Wireless local area networks (WLANs) provide network connectivity with portability, allowing wireless network connectivity for devices such as computers, personal digital assistants, wireless phones, and other devices generally referred to as wireless stations. A wireless station, can move about within range of a WLAN base station, referred to as an access point, which typically connects to a wired network and acts as a gateway between wireless stations and the wired network. Any number of applications can be supported over a WLAN, including simple internet access to streaming real time data, such as video and voice calling.
Access points serve as the master timing source for the wireless stations. Each wireless station associated with the access point must synchronize to that access point's timer. To facilitate synchronization, access points broadcast beacon signals, or simply beacons. Beacons contain information about the state of the access point's timer so that wireless stations can adjust their own timer to run in synchronization with the access point. Being in synchronization allows the wireless stations to place portions of the WLAN circuitry into a low power or sleep state, and wake up in time to receive information from the access point at periodic intervals, if necessary. Beacons also allow wireless stations to determine the quality of the signal received from the access point, and compare it with neighboring access points to determine if a change in association is necessary.
Multiple organizations may operate access points within range of each other on the same WLAN channel. When two access points within range of each other are transmitting on the same channel, timing issues can arise due to the imprecision in access point timers. Although they may be programmed to transmit beacons at the same regular interval, differences in the tolerance of timing circuits can cause a difference in time keeping among access points. The result is that one access point's beacons, which initially may have been offset in time with another access point's beacons on the same channel, may begin to “catch up” and overlap, and eventually “pass” the beacons of the other access point.
WLANs may be configured to operate as a closed or open system. In an open WLAN, the organization's Service Set Identifier (SSID) is included in the beacon or supplemental beacon transmissions. Wireless stations can easily determine whether or not a beacon or supplemental beacon transmission belongs to a desired organization. In a closed WLAN, the organization's SSID is not included in the beacon or supplemental beacon transmissions. Wireless stations can only identify which beacons or supplemental beacons belong to a desired organization by parsing the Basic Service Set Identification (BSSID) from the Medium Access Control (MAC) header of the frame. The wireless station then compares the BSSID from the beacon or supplemental beacon to a list of neighbor access points which belong to the desired organization.
Wireless stations may utilize passive scanning for obtaining measurements of a neighbor access point's signal strength. As defined by the WLAN protocol, passive scanning is a simple method whereby the wireless station enables the receiver for a specified duration on a channel to search for beacon or supplemental beacon transmissions. When the passive scan has been completed, the beacons and/or supplemental beacons are processed to determine which belong to the wireless station's neighbor access points. Since the wireless station may be in range of multiple WLANs, there may be beacon and/or supplemental beacon transmissions which belong to other organizations.
Wireless stations may also employ an enhanced passive scanning technique which is based on the WLAN protocol defined passive scanning technique. Enhanced passive scanning allows a wireless station to minimize the amount of time the wireless station's receiver is enabled by calculating the target times at which beacon and/or supplemental beacon transmissions are expected to be transmitted. The wireless station uses the currently associated access point and neighbor access point timing information to calculate the target time of a beacon or supplemental beacon transmission from a neighbor access point. To obtain a neighbor access point measurement, the wireless station enables the receiver at a specific moment in time and when the beacon and/or supplemental beacon is received the receiver is immediately disabled. When multiple access points are operating on the same channel, the wireless station may come out of power save mode at a target time, and receive the wrong access point's beacon. Therefore there is a need for a means by which wireless stations can effectively use power save operation and still receive the correct beacon when there is more than one access point operating on the presently associated channel.